It depends. If there is specific wording on the rental contract allowing your landlord to store items on the property during your occupation, then he may be within his rights. As an example, I own a home on property with a large storage shed at the rear of the property. In my rental contract, it states that I have to right to store my belonging within the shed, and to access it at any time. My tenants signed the rental agreement with this knowledge, and it has not been an issue for either party.
I suggest that you review the contract to make sure both parties are sticing to the agreement. You may also want to speak nicely with your landlord to clarify what has been happening and find out if this will be an ongoing situation. If so, and there is nothing in the rental contract that address his use of the property while you live there, you may want to try and renegotiate the lease, or contact a real estate attorney for further advice. Best of luck!